The primary focus of this project is the investigation of the relative roles of mutation, recombination, genetic drift and natural selection in shaping the levels of genetic variation observed at the DNA level. Several experiments address the fundamental question: what is the quantity and quality of molecular population genetic variation? To obtain a general answer many loci (white, yellow to achete, g-6-phd, forked, vermilion, suppressor of forked and zeste) in natural populations of Drosophila have been surveyed. A specific question in these and comparative studies with other species is the consequence of large differences in the amounts of crossing over per kilobase on the molecular genetic variation. The experimental results in conjunction with theoretical studies suggest that reduced levels of DNA sequence polymorphism in chromosome regions where crossing over is reduced are caused by the "hitch-hiking" effect of rare selectively favored and linked mutants. A new aspect of this program that grew out of previous work examining transposons in D. ananassae is the cytological study of male meiosis. This species shows regular meiotic crossingover in males, which is atypical of other closely related Drosophila species. This project except for the cytological analysis of D. ananassae male meiosis was terminated in January 1990. Major aspects of this research continue to be investigated by the PI and others of this group in their new positions. This project is continuing as a collaboration between Dr. Langley and the Judd group at NIEHS (see Z01 ES 65037-06 LMG). The light and electron microscopic examination of meiosis in D. ananassae males has produced some figures of cells in early prophase. Detailed examination of the electron microscope pictures should answer questions about crucial structures such as the synaptonemal complex.